On the origin of white dwarfs with carbon-dominated atmospheres: the case of H1504+65

¹ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, (1900) La Plata, Argentina e-mail: [althaus;acorsico]@fcaglp.unlp.edu.ar
² Instituto de Astrofísica La Plata, IALP, CONICET-UNLP, Argentina
³ Member of the Carrera del Investigador Científico y Tecnológico, CONICET, Argentina
⁴ Departament de Física Aplicada, Escola Politècnica Superior de Castelldefels, Universitat Politècnica de Catalunya, Av. del Canal Olímpic, s/n, 08860 Castelldefels, Spain e-mail: [santi;garcia]@fa.upc.edu
⁵ Institute for Space Studies of Catalonia, c/Gran Capità 2–4, Edif. Nexus 104, 08034 Barcelona, Spain

Received: 6 August 2008
Accepted: 12 November 2008

Abstract

Aims. We explore different evolutionary scenarios to explain the helium deficiency observed in H1504+65, the most massive known PG1159 star.

Methods. We concentrate mainly on the possibility that this star could be the result of mass loss shortly after its born-again and during its subsequent evolution through the [WCL] stage. This possibility is consistent with observational evidence of extensive mass-loss events in Sakurai's object and is in line with the finding that these mass losses give rise to PG1159 models with thin helium-rich envelopes and high rates of period change, as demanded by the pulsating star PG1159-035. We compute the post-born-again evolution of massive sequences by taking into account different mass-loss rate histories.

Results. Our results show that stationary winds during the post-born-again evolution fail to remove completely the helium-rich envelope and are therefore unable to explain the helium deficiency observed in H1504+65. Stationary winds during the Sakurai and [WCL] stages remove at most half of the envelope that survives the violent hydrogen burning during the born-again phase.

Conclusions. In view of our results, the proposed evolutionary sequence of born-again stars to H1504+65 and then to white dwarfs with carbon-rich atmospheres is difficult to reproduce unless the entire helium-rich envelope is ejected by non-stationary mass-loss episodes during the Sakurai stage.